Aircraft having Radially Extendable Tailboom Assembly

ABSTRACT

A tail sitter aircraft includes a fuselage having a forward portion, an aft portion and a longitudinally extending fuselage axis. At least two wings are supported by the forward portion of the fuselage. A distributed propulsion system includes at least one propulsion assembly operably associated with each fixed wing and is operable to provide forward thrust during forward flight and vertical thrust during vertical takeoff, hover and vertical landing. A tailboom assembly extends from the aft portion of the fuselage and includes a plurality of rotatably mounted tail arms having control surfaces and landing members. In a forward flight configuration, the tail arms are radially retracted to reduce tail surface geometry and provide yaw and pitch control with the control surfaces. In a landing configuration, the tail arms are radially extended relative to the fuselage axis to form a stable ground contact base with the landing members.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of co-pending application Ser.No. 15/241,436 filed Aug. 19, 2016.

TECHNICAL FIELD OF THE DISCLOSURE

The present disclosure relates, in general, to aircraft that take offand land on their tail and tilt horizontally for forward flight and, inparticular, to tail sitter aircraft having a main lifting surfaceforward of a tailboom assembly that radially extends to form landinggear having a stable ground contact base.

BACKGROUND

Fixed-wing aircraft, such as airplanes, are capable of flight usingwings that generate lift responsive to the forward airspeed of theaircraft, which is generated by thrust from one or more jet engines orpropellers. The wings generally have an airfoil cross section thatdeflects air downward as the aircraft moves forward, generating the liftforce to support the airplane in flight. Fixed-wing aircraft, however,typically require a runway that is hundreds or thousands of feet longfor takeoff and landing.

Unlike fixed-wing aircraft, vertical takeoff and landing (VTOL) aircraftdo not require runways. Instead, VTOL aircraft are capable of takingoff, hovering and landing vertically. One example of a VTOL aircraft isa helicopter which is a rotorcraft having one or more rotors thatprovide lift and thrust to the aircraft. The rotors not only enablehovering and vertical takeoff and landing, but also enable, forward,backward and lateral flight. These attributes make helicopters highlyversatile for use in congested, isolated or remote areas wherefixed-wing aircraft may be unable to takeoff and land. Helicopters,however, typically lack the forward airspeed of fixed-wing aircraft.

A tiltrotor aircraft is another example of a VTOL aircraft. Tiltrotoraircraft generate lift and propulsion using proprotors that aretypically coupled to nacelles mounted near the ends of a fixed wing. Thenacelles rotate relative to the fixed wing such that the proprotors havea generally horizontal plane of rotation for vertical takeoff, hoveringand landing and a generally vertical plane of rotation for forwardflight, wherein the fixed wing provides lift and the proprotors provideforward thrust. In this manner, tiltrotor aircraft combine the verticallift capability of a helicopter with the speed and range of fixed-wingaircraft. Tiltrotor aircraft, however, typically suffer from downwashinefficiencies during vertical takeoff and landing due to interferencecaused by the fixed wing.

A further example of a VTOL aircraft is a tiltwing aircraft thatfeatures a rotatable wing that is generally horizontal for forwardflight and rotates to a generally vertical orientation for verticaltakeoff and landing. Propellers are coupled to the rotating wing toprovide the required vertical thrust for takeoff and landing and therequired forward thrust to generate lift from the wing during forwardflight. The tiltwing design enables the slipstream from the propellersto strike the wing on its smallest dimension, thus improving verticalthrust efficiency as compared to tiltrotor aircraft. Tiltwing aircraft,however, are more difficult to control during hover as the verticallyoriented wing provides a large surface area for crosswinds, typicallyrequiring tiltwing aircraft to have either cyclic rotor control or anadditional thrust station to generate a moment.

Another example of a VTOL aircraft is a tail sitter aircraft that landsand takes off on its tail section. The longitudinal fuselage axis isgenerally horizontal during forward flight and is generally vertical forhover, takeoff and landing. A fixed propulsion system is typically usedto generate vertical thrust during hover, takeoff and landing andhorizontal thrust during forward flight, wherein the wings provide lift.It has been found, however, the tail sitter aircraft having forwardwings are unstable on the ground due to a high center of gravity and alarge wing surface exposed to wind. Attempts have been made to designtail sitter aircraft with aft wings to lower the center of gravity whilethe aircraft is on the ground. It has been found, however, that havingan aft main lifting surface reduces the aerodynamic stability of theaircraft during forward flight.

SUMMARY

In a first aspect, the present disclosure is directed to a tail sitteraircraft including a fuselage having a forward portion, an aft portionand a longitudinally extending fuselage axis. A main lifting surface issupported by the forward portion of the fuselage. A propulsion system isoperably associated with the main lifting surface and is operable toprovide thrust during forward flight, vertical takeoff, hover andvertical landing. A tailboom assembly extends from the aft portion ofthe fuselage. The tailboom assembly includes a plurality of rotatablymounted tail arms having control surfaces and landing members wherein,in a forward flight configuration, the tail arms are radially retractedto reduce tail surface geometry and provide yaw and pitch control withthe control surfaces and, wherein, in a landing configuration, the tailarms are radially extended relative to one another about the fuselageaxis to form a stable ground contact base with the landing members.

In some embodiments, the main lifting surface may include a pair ofgenerally oppositely disposed wings. In other embodiments, the mainlifting surface may include at least three generally circumferentiallydistributed wings. In certain embodiments, the propulsion system may bea distributed propulsion system such as a propulsion system including aplurality of independently controllable cross-flow fans includingindependently controllable variable thrust cross-flow fans. In otherembodiments, the propulsion system may include at least a pair of rotorassemblies including independently controllable rotor assemblies.

In some embodiments, the tailboom assembly may include at least threerotatably mounted tail arms. In other embodiments, the tailboom assemblymay include at least four rotatably mounted tail arms. In certainembodiments, the tailboom assembly may have an actuator assemblyoperable to transition the tail arms between the forward flightconfiguration and the landing configuration. The actuator assembly mayinclude a cross arm assembly coupled to each of the tail arms. Theactuator assembly may also include one or more linear actuators, one ormore rotary actuators or combinations thereof. In addition, the actuatorassembly may include a locking system operable to secure the tail armsin one or both of the forward flight configuration and the landingconfiguration.

In a second aspect, the present disclosure is directed to a tail sitteraircraft including a fuselage having a forward portion, an aft portionand a longitudinally extending fuselage axis. First and second wings aresupported by the forward portion of the fuselage and provide a mainlifting surface for the aircraft. A propulsion system includes first andsecond rotor assemblies that are respectively attached to the first andsecond wings. The propulsion system is operable to provide thrust duringforward flight, vertical takeoff, hover and vertical landing. A tailboomassembly extends from the aft portion of the fuselage. The tailboomassembly includes a plurality of rotatably mounted tail arms havingcontrol surfaces and landing members wherein, in a forward flightconfiguration, the tail arms are radially retracted to reduce tailsurface geometry and provide yaw and pitch control with the controlsurfaces and, wherein, in a landing configuration, the tail arms areradially extended relative to one another about the fuselage axis toform a stable ground contact base with the landing members.

In a third aspect, the present disclosure is directed to a tail sitteraircraft including a fuselage having a forward portion, an aft portionand a longitudinally extending fuselage axis. At least three generallycircumferentially distributed wings are supported by the forward portionof the fuselage and provide a main lifting surface for the aircraft. Adistributed propulsion system is operably associated with the wings andis operable to provide thrust during forward flight, vertical takeoff,hover and vertical landing. A tailboom assembly extends from the aftportion of the fuselage. The tailboom assembly includes a plurality ofrotatably mounted tail arms having control surfaces and landing memberswherein, in a forward flight configuration, the tail arms are radiallyretracted to reduce tail surface geometry and provide yaw and pitchcontrol with the control surfaces and, wherein, in a landingconfiguration, the tail arms are radially extended relative to oneanother about the fuselage axis to form a stable ground contact basewith the landing members.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent disclosure, reference is now made to the detailed descriptionalong with the accompanying figures in which corresponding numerals inthe different figures refer to corresponding parts and in which:

FIGS. 1A-1B are schematic illustrations of a tail sitter aircraft inaccordance with embodiments of the present disclosure;

FIGS. 1C-1D are schematic illustrations of a variable thrust cross-flowfan system for a tail sitter aircraft in accordance with embodiments ofthe present disclosure;

FIGS. 2A-2F are schematic illustrations of a tail sitter aircraft in asequential flight operating scenario in accordance with embodiments ofthe present disclosure;

FIGS. 3A-3B are schematic illustrations of a tail sitter aircraft inaccordance with embodiments of the present disclosure; and

FIGS. 4A-4D are various views of a tailboom assembly for a tail sitteraircraft in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

While the making and using of various embodiments of the presentdisclosure are discussed in detail below, it should be appreciated thatthe present disclosure provides many applicable inventive concepts,which can be embodied in a wide variety of specific contexts. Thespecific embodiments discussed herein are merely illustrative and do notdelimit the scope of the present disclosure. In the interest of clarity,not all features of an actual implementation may be described in thepresent disclosure. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would be a routine undertakingfor those of ordinary skill in the art having the benefit of thisdisclosure.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, and the like described herein may be positioned inany desired orientation. Thus, the use of terms such as “above,”“below,” “upper,” “lower” or other like terms to describe a spatialrelationship between various components or to describe the spatialorientation of aspects of such components should be understood todescribe a relative relationship between the components or a spatialorientation of aspects of such components, respectively, as the devicedescribed herein may be oriented in any desired direction.

Referring to FIGS. 1A-1B in the drawings, a tail sitter aircraft isschematically illustrated and generally designated 10. Tail sitteraircraft 10 includes a fuselage 12, a plurality of wings 14, 16, 18 anda tailboom assembly 20. As illustrated, wings 14, 16, 18 are forward oftailboom assembly 20 during forward flight and are considered to besupported by a forward portion of fuselage 12 while tailboom assembly 20extends from an aft portion of fuselage 12. Preferably, wings 14, 16, 18each have an airfoil cross-section operable to generate lift responsiveto the forward airspeed of tail sitter aircraft 10 and form the mainlifting surface of tail sitter aircraft 10. Tail sitter aircraft 10 hasa propulsion system depicted as a distributed propulsion system 22including a plurality of propulsion assemblies in the form of cross-flowfans located in chordwise channels of wings 14, 16, 18. For example,cross-flow fans 22A-22E are located in chordwise channels 24A-24Edepicted in phantom in wing 16 in FIG. 1B.

Preferably, as discussed herein, the cross-flow fans have variablethrust capacities and may be operated independent of one another.Cross-flow fans of the present disclosure may be operated responsive toone or more electrical motors, hydraulic motors and/or liquid fuelpowered engines. One or more cross-flow fans may be operated on a commondrive shaft or each cross-flow fan may be operated by a unique drivesystem. Preferably, variable thrust control, as discussed herein, foreach cross-flow fan is independent. Alternatively, more than onecross-flow fan could share a common variable thrust control actuator orsystem. As illustrated, air enters cross-flow fans 22A-22E from aforward intake portion of a respective chordwise channel 24A-24E andexits cross-flow fans 22A-22E into an aft discharge portion of arespective chordwise channel 24A-24E, thereby generating thrustgenerally parallel to a longitudinal fuselage axis 26. The aft portionsof chordwise channels may include flaperons or other flow directingmembers to enable thrust vectoring.

Tailboom assembly 20 includes a plurality of control surfaces usedduring forward flight depicted as rudder 28 for yaw control andelevators 30, 32 for pitch control, in the illustrated configuration. Itis noted that tail sitter aircraft 10 may fly in other orientationswherein the control surfaces may serve alternate functions. For example,if wings 16, 18 are above fuselage 12 and wing 14 is below fuselage 12,then control surface 32 would operate as the rudder and control surfaces28, 30 would operate as the elevators. In addition, even though tailsitter aircraft 10 is depicted and described as having three wings thatare circumferentially distributed uniformly about fuselage 12, it shouldbe understood by those skilled in the art that a tail sitter aircraft ofthe present disclosure could have other numbers of wings both greaterthan and less than three and/or have wings that are oriented in anonuniform manner. In the present example, instead of wings 14, 16, 18being oriented at 120-degree circumferential intervals (120/120/120),the wings could be oriented as 105/150/105, 90/180/90 or other desiredwing orientation permutation. Also, even though control surface 28, 30,32 are depicted as being circumferentially offset from wings 14, 16, 18by 60 degrees, it should be understood by those skilled in the art thatcontrol surfaces for a tail sitter aircraft of the present disclosurecould have other orientations relative to the wings including beingcircumferentially inline with the wings. Further, even though the samenumber of wings and control surfaces has been depicted, it should beunderstood by those skilled in the art that the number of controlsurfaces and the number of wings are independent of each other.

Tailboom assembly 20 includes a plurality of tail arms 34, 36, 38 thatare operable to be radially retracted in forward flight, as best seen inFIG. 1A, forming a small tail surface geometry wherein control surfaces28, 30, 32 provide yaw and pitch control. In addition, tail arms 34, 36,38 are operable to be radially extended for landing, as best seen inFIG. 1B, forming a stable ground contact base. As illustrated, each tailarm 34, 36, 38 includes one of the control surface 28, 30, 32. Asillustrated, each tail arm 34, 36, 38 includes a landing member 40, 42,44 such as a fixed or retractable skid member or shock absorbing membersuch as a pneumatic shock strut or mechanical spring assembly. Landingmembers 40, 42, 44 may also include wheels to assist in groundmaneuvers. The length of tail arms 34, 36, 38 as well as the angle tailarms 34, 36, 38 make with longitudinal fuselage axis 26 in the landingconfiguration may be determined based upon the location of the center ofgravity 46 of tail sitter aircraft 10. Preferably, center of gravity 46should be located within the tip over angle from the ground contact base48 of landing member 40, 42, 44, wherein the tip over angle may be about55 degrees. As discussed herein, tailboom assembly 20 includes anactuator assembly 50 operable to transition tail arms 34, 36, 38 betweenthe forward flight configuration and the landing configuration. Tailarms 34, 36, 38 may also include one more sensors that indicate theposition of tail arms 34, 36, 38 such as fully retracted and fullydeployed as well as fault positions if tail arms 34, 36, 38 fail toreach the fully retracted and/or the fully deployed positions. Tail arms34, 36, 38 may also include a fail safe mechanism to bias tail arms 34,36, 38 toward the landing configuration in the event of a tail armfault.

Referring to FIG. 1C, therein is depicted an embodiment of a variablethrust cross-flow fan, such as variable thrust cross-flow fan 22A ofFIG. 1B. Variable thrust cross-flow fan 22A includes a cross-flow fanassembly 52 including driver plates 54, 56 which are coupled to and arerotatable about a longitudinal axis by a drive shaft 58. Cross-flow fanassembly 52 includes a plurality of blades 58 that are each rotatablycoupled between driver plates 54, 56. As illustrated, blades 58 aredisposed radially outwardly from the longitudinal axis such that blades58 follow a generally circular path of travel when cross-flow fanassembly 52 rotates about the longitudinal axis. Variable thrustcross-flow fan 22A includes a control assembly 60 that is coupled toeach of blades 58. In the illustrated embodiment, control assembly 60includes a control cam 62 that is rotatable with and translatablerelative to cross-flow fan assembly 52. Control assembly 60 alsoincludes a plurality of linkages 64 that are slidably coupled to controlcam 62 via follower pins 66 in follower slots 68 of driver plate 54 andfixably coupled to blades 58 via driver pins 70 that extend throughlinkage holes (not visible) of driver plate 54.

When cross-flow fan assembly 52 is rotated by drive shaft 58 and controlcam 62 is positioned concentrically with cross-flow fan assembly 52,follower pins 66 do not move relative to follower slots 68 and blades 58do not rotate relative to driver plates 54, 56. In this state, blades 58are in a neutral configuration wherein each of the blades 58 has asubstantially zero pitch during an entire revolution of cross-flow fanassembly 52, as illustrated in FIG. 1C. In this neutral configuration,all of blades 58 have a substantially zero angle of attack and thereforeproduce little or no thrust. To produce thrust, blades 58 are rotatedrelative to driver plates 54, 56 in response to shifting control cam 62forward, in the chordwise direction of wing 16, relative to cross-flowfan assembly 52. When control cam 62 is position eccentrically relativeto cross-flow fan assembly 52 and cross-flow fan assembly 52 is rotatedby drive shaft 58, follower pins 66 cyclically slide within followerslots 68 which cyclically pivots linkages 64 and cyclically rotatesblades 58 relative to driver plates 54, 56.

Referring additionally to FIG. 1D, variable thrust cross-flow fan 22A isdisposed within chordwise channel 24A of wing 16. In the illustratedconfiguration, the center of rotation of control cam 62 has been shiftedforward from a concentric location 72 to an eccentric location 74. Inthis configuration, as each blade 58 follows the generally circular pathof travel, the blades transition between positive pitch, zero pitch,negative pitch, zero pitch and back to positive pitch during eachrevolution of cross-flow fan assembly 52. As illustrated, blades 58 havean airfoil cross section and travel in a counterclockwise direction. Asblades 58 approach forward intake 76 of chordwise channel 24A, theblades have progressively greater positive pitch reaching a maximumpositive pitch proximate axis 78. Thereafter, as blades 58 retreat fromforward intake 76, the blades have progressively lesser positive pitchreaching zero pitch proximate axis 80. As blades 58 approach aftdischarge 82 of chordwise channel 24A, the blades have progressivelygreater negative pitch reaching a maximum negative pitch proximate axis78. Thereafter, as blades 58 retreat from aft discharge 82, the bladeshave progressively lesser negative pitch, reaching zero pitch proximateaxis 80. Each blade 58 repeats this cycle on each revolution ofcross-flow fan assembly 52.

As blades 58 follow the generally circular path of travel with thecyclically varying angle of attack described herein, air passes throughcross-flow fan assembly 52 as indicated by low-density intake airflowarrows 84 and high-density discharge airflow arrow 86 with the resultantthrust indicated by arrow 88. The magnitude of thrust 88 generated byvariable thrust cross-flow fan system 22A is determined by factorsincluding the magnitude of the eccentricity applied to control cam 62,the rotational speed of cross-flow fan assembly 52, the cross sectionalshape of blades 58, the pitch cycle of blades 58, the number of blades58 and other factors known to those having skill in the art.

Referring next to FIGS. 2A-2F in the drawings, a sequentialflight-operating scenario of tail sitter aircraft 10 is depicted. Tailsitter aircraft 10 may be a manned or unmanned aircraft and may beoperated responsive to onboard pilot flight control, remote flightcontrol or autonomous flight control. Tail sitter aircraft 10 ispreferably a fly-by-wire aircraft including an onboard flight controlcomputing system that is operable to receive sensor data from and sendflight commands to propulsion system controllers, flaperon controllers,control surface controllers, tailboom actuator assembly controllers,landing member actuator controllers, other systems controllers and thelike. Preferably, onboard flight control computing system is operable toindividually and independently control and operate each of thepropulsion assemblies. As best seen in FIG. 2A, tail sitter aircraft 10is in its vertical takeoff and landing configuration. Preferably, allpropulsion assemblies are operating to provide maximum thrust andcontrol during vertical takeoff and hover operations. As tail sitteraircraft 10 continues its vertical assent to a desired elevation, it maybegin the transition from the vertical takeoff and landing configurationtoward the forward flight configuration.

As best seen in FIG. 2B, as tail sitter aircraft 10 transitions fromvertical takeoff and landing to forward flight, tailboom actuatorassembly 50 radially retracts tail arms 34, 36, 38 such that tailboom 20has a reduced tail surface geometry. Tail sitter aircraft 10 also beginsto transition its longitudinal fuselage axis 26 from the verticalattitude toward the horizontal attitude. As best seen in FIG. 2C, tailsitter aircraft 10 has completed the transition to forward flight mode.During forward flight, it may be desirable to maximize flightefficiency, which in turn increases the endurance of tail sitteraircraft 10. One way to increase efficiency is to fly tail sitteraircraft 10 with a single wing, in this case wing 18, in the downposition and with wings 14, 16 extending upwardly at approximately 30degrees relative to a horizontal axis. In the illustrated embodiment,this position results in desirably oriented control surfaces depicted asrudder 28 for yaw control and elevators 30, 32 for pitch control. Inaddition, once tail sitter aircraft 10 is in the forward flight mode,the thrust requirements are reduced compared to the thrust requirementsof vertical takeoff and hovering. Accordingly, in forward flight mode,the thrust output of one or more of the propulsion assemblies may bereduced by, for example, reducing the eccentricity of control cam 62relative to cross-flow fan assembly 52. Alternatively or additionally,one or more of the propulsion assemblies may be shut down during forwardflight. It is noted that during forward flight, aerodynamic forces tendto bias tail arms 34, 36, 38 toward the radially retracted, forwardflight configuration.

Continuing with the current example, as tail sitter aircraft 10approaches the destination, all propulsion assemblies are preferablyreengaged to provide full propulsion capabilities while remaining inforward flight mode, as best seen in FIG. 2D. Thereafter, tail sitteraircraft 10 may begin its transition from forward flight mode tovertical takeoff and landing mode, as best seen in FIG. 2E, whereinlongitudinal fuselage axis 26 shifts from the horizontal attitude towardthe vertical attitude. As tail sitter aircraft 10 continues its verticaldescent, as best seen in FIG. 2F, tailboom actuator assembly 50 radiallyextends tail arms 34, 36, 38 such that tailboom 20 forms a stable groundcontact base. In addition, tailboom actuator assembly 50 may deploylanding members 40, 42, 44, if they were retracted during forward flightmode.

Referring to FIGS. 3A-3B in the drawings, a tail sitter aircraft isschematically illustrated and generally designated 100. Tail sitteraircraft 100 includes a fuselage 102, a pair of wings 104, 106 and atailboom assembly 108. As illustrated, wings 104, 106 are forward oftailboom assembly 108 during forward flight and are considered to besupported by a forward portion of fuselage 102 while tailboom assembly108 extends from an aft portion of fuselage 102. Preferably, wings 104,106 each have an airfoil cross-section operable to generate liftresponsive to the forward airspeed of tail sitter aircraft 100 and formthe main lifting surface of tail sitter aircraft 100. Tail sitteraircraft 100 includes a pair of propulsion assemblies depicted as rotorassemblies 110, 112. Rotor assembly 110 includes a fixed nacelle 114that houses an engine and transmission that provide torque androtational energy to a drive shaft that rotates a rotor hub assembly 116and a plurality of rotor blade assemblies 118. Likewise, rotor assembly112 includes a fixed nacelle 120 that houses an engine and transmissionthat provide torque and rotational energy to a drive shaft that rotatesa rotor hub assembly 122 and a plurality of rotor blade assemblies 124.

Even though tail sitter aircraft 100 is depicted as having a particularnumber of rotor assemblies, it should be understood by those skilled inthe art that a tail sitter aircraft of the present disclosure could haveany desired number of rotor assemblies, wherein the operation of therotor assemblies may be independent. Likewise, even though rotorassemblies 110, 112 are depicted as having a particular number of rotorblade assemblies, it should be understood by those skilled in the artthat rotor assemblies of the present disclosure could have any desirednumber of rotor blade assemblies. Also, even though rotor bladeassemblies 118, 124 are depicted as having a particular length andtwist, it should be understood by those skilled in the art that rotorblade assemblies of the present disclosure could have any desiredconfiguration suitable for providing vertical thrust in landingconfiguration and forward thrust in forward flight configuration.

Tailboom assembly 108 includes a plurality of control surfaces usedduring forward flight depicted as rudder 126 for yaw control andelevators 128, 130 for pitch control. Tailboom assembly 108 alsoincludes a plurality of tail arms 132, 134, 136, 138 that are operableto be radially retracted in forward flight, as best seen in FIG. 3A,forming a small tail surface geometry wherein control surfaces 128, 130,132 provide yaw and pitch control. In addition, tail arms 132, 134, 136,138 are operable to be radially extended for landing, as best seen inFIG. 3B, forming a stable ground contact base. As illustrated, tail arm132 includes rudder 126, tail arm 134 includes elevator 128 and tail arm138 includes elevator 130. Also, each tail arm 132, 134, 136, 138includes a landing member 140, 142, 144, 146, illustrated as pneumaticshock struts with wheels. The length of tail arms 132, 134, 136, 138 aswell as the angle tail arms 132, 134, 136, 138 make with a longitudinalfuselage axis 148 in the landing configuration may be determined basedupon the location of the center of gravity 150 of tail sitter aircraft100. Preferably, center of gravity 150 should be located within the tipover angle from the ground contact base 152 of landing members 140, 142,144, 146. As discussed herein, tailboom assembly 108 includes anactuator assembly 154 operable to transition tail arms 132, 134, 136,138 between the forward flight configuration and the landingconfiguration.

Referring next to FIGS. 4A-4D of the drawings, various views of atailboom assembly for a tail sitter aircraft are illustrated andgenerally designated 200. Tailboom assembly 200 includes a mountingassembly 202 that is operable to be connected to an aft portion of thefuselage of a tail sitter aircraft by bolting, pining, threading,welding or other suitable coupling technique. In the illustratedembodiment, tailboom assembly 200 includes three tail arms 204, 206,208. As discussed herein, tail arms 204, 206, 208 are operable to beradially retracted in a forward flight mode to form a small tail surfacegeometry wherein control surfaces 210, 212, 214 provide yaw and pitchcontrol. In addition, tail arms 204, 206, 208 are operable to beradially extended in a landing mode to form a stable ground contact basewith landing members 216, 218, 220. Mounting assembly 202 includes aframe member 222 that may be a component of tailboom assembly 200 or acomponent of the aft portion of the fuselage of a tail sitter aircraft,depending upon the implementation. Mount assembly 202 also includes atail arm attachment fitting 224 that is securably coupled to framemember 222 by three brackets 226, 228, 230. In the illustratedembedment, brackets 226, 228, 230 are bolted to frame member 222 andtail arm attachment fitting 224, however, those skilled in the art willrecognize that brackets of the present disclosure may be secured toframe members and tail arm attachment fittings of the present disclosureusing other coupling techniques including pins, rivets, welding,adhesion and the like. Alternatively, brackets of the present disclosurecould be integral with frame members and/or tail arm attachment fittingsof the present disclosure. Tail arm attachment fitting 224 has aplurality of flanges 232, 234, 236, 238, 240, 242, each having aneyehole (not visible) for receiving a fastener therethrough.

Tail arms 204, 206, 208 each include a dual clevis bracket assembly 244,246, 248 attached to a forward end thereof. In the illustratedembodiment, dual clevis bracket assemblies 244, 246, 248 are bolted totail arms 204, 206, 208, however, those skilled in the art willrecognize that dual clevis bracket assemblies of the present disclosuremay be secured to tail arms of the present disclosure using othercoupling techniques including pins, rivets, welding, adhesion and thelike. As illustrated, dual clevis bracket assembly 244 mates withflanges 232, 234 of tail arm attachment fitting 224 and is rotatablycoupled therewith using bolt connections. Likewise, dual clevis bracketassembly 246 mates with flanges 236, 238 of tail arm attachment fitting224 and is rotatably coupled therewith using bolt connections and dualclevis bracket assembly 248 mates with flanges 240, 242 of tail armattachment fitting 224 and is rotatably coupled therewith using boltconnections. Even though dual clevis bracket assemblies 244, 246, 248are depicted as being rotatably coupled to tail arm attachment fitting224 using bolt connections, it should be understood by those skilled inthe art that dual clevis bracket assemblies of the present disclosuremay be rotatably coupled to tail arm attachment fitting of the presentdisclosure using other coupling techniques including pins, bearings andthe like.

The illustrated mounting assembly 202 enables tail arms 204, 206, 208 torotate relative to the fuselage of a tail sitter aircraft such that tailarms 204, 206, 208 are operable to radially retract in a forward flightconfiguration and radially extend in a landing configuration, asdiscussed herein. Even though mounting assembly 202 has been depictedand described as having a particular array of components in a particularconfiguration, it should be understood by those skilled in the art thata mounting assembly of the present disclosure may have fewer components,more components and/or different components.

Tailboom assembly 200 includes an actuator assembly 250 operable totransition tail arms 204, 206, 208 between the forward flightconfiguration and the landing configuration. In the illustratedembodiment, actuator assembly 250 includes three brace arms 252, 254,256 that are rotatably mounted to tail arms 204, 206, 208 at outersupport members, only outer support member 258 of tail arm 208 beingvisible in the drawings. In addition, brace arms 252, 254, 256 arerotatably coupled to a central support member 260 that guilds movementof brace arms 252, 254, 256 during transitions between the forwardflight configuration and the landing configuration. Preferably, bracearms 252, 254, 256 form a truss structure between tail arms 204, 206,208 when the tail sitter aircraft is on the ground to reduce or preventbending moments in tail arms 204, 206, 208. In the illustratedembodiment, brace arms 252, 254, 256 are moved together as a unit byactuator 262, thereby rotating tail arms 204, 206, 208 together duringtransitions between the forward flight configuration and the landingconfiguration. Preferably, actuator 262 is an electrically operatedactuator that may be a linear actuator, a rotary actuator or acombination thereof. As illustrated, actuator 262 is supported by a tailarm, namely tail arm 204, but could alternatively be supported bymounting assembly 202 or another component of the tail sitter aircraft.

Actuator 262 includes a brake system operable to secure actuator 262 indesired positions, thereby enabling actuator 262 to serve as a lockagainst unwanted rotational movement of tail arms 204, 206, 208. Forexample, it may be desirable to lock tail arms 204, 206, 208 in theforward flight configuration and/or the landing configuration forsecurity and safety. Alternatively or additionally, a locking systemseparate from actuator 262 may be used to secure tail arms 204, 206, 208in the forward flight configuration and/or the landing configuration.Even though a single actuator 262 has been depicted and described fortransitioning tail arms 204, 206, 208 between the forward flightconfiguration and the landing configuration, it should be understood bythose skilled in the art that tail arm transitioning for a tail sitteraircraft of the present disclosure could utilize other actuationprotocols including having individual actuators for each tail arm. Inaddition, actuator 262 or another actuation system may be used to deploylanding members 216, 218, 220 in embodiments having retractable landingmembers.

The foregoing description of embodiments of the disclosure has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the disclosure to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the disclosure. Theembodiments were chosen and described in order to explain the principalsof the disclosure and its practical application to enable one skilled inthe art to utilize the disclosure in various embodiments and withvarious modifications as are suited to the particular use contemplated.Other substitutions, modifications, changes and omissions may be made inthe design, operating conditions and arrangement of the embodimentswithout departing from the scope of the present disclosure. Suchmodifications and combinations of the illustrative embodiments as wellas other embodiments will be apparent to persons skilled in the art uponreference to the description. It is, therefore, intended that theappended claims encompass any such modifications or embodiments.

1. A tail sitter aircraft comprising: a fuselage having a forwardportion, an aft portion and a longitudinally extending fuselage axis; atleast two fixed wings supported by the forward portion of the fuselage,the fixed wings providing a main lifting surface for the aircraft; adistributed propulsion system including at least one propulsion assemblyoperably associated with each fixed wing, the distributed propulsionsystem operable to provide forward thrust during forward flight andvertical thrust during vertical takeoff, hover and vertical landing; anda tailboom assembly extending from the aft portion of the fuselage, thetailboom assembly including a plurality of rotatably mounted tail armshaving control surfaces and landing members; wherein, in a forwardflight configuration, the tail arms are radially retracted to reducetail surface geometry and provide yaw and pitch control with the controlsurfaces; and wherein, in a landing configuration, the tail arms areradially extended relative to one another about the fuselage axis toform a stable ground contact base with the landing members.
 2. The tailsitter aircraft as recited in claim 1 wherein the at least two fixedwings further comprise first and second fixed wings that are generallyoppositely disposed relative to the fuselage.
 3. The tail sitteraircraft as recited in claim 1 wherein the at least two fixed wingsfurther comprise fixed wings that are nonrotatable relative to thefuselage.
 4. The tail sitter aircraft as recited in claim 1 wherein eachpropulsion system further comprises a rotor assemblies.
 5. The tailsitter aircraft as recited in claim 1 wherein each propulsion systemfurther comprises a cross-flow fan.
 6. The tail sitter aircraft asrecited in claim 1 wherein each propulsion system further comprises avariable thrust cross-flow fans.
 7. The tail sitter aircraft as recitedin claim 1 wherein the tailboom assembly further comprises at leastthree rotatably mounted tail arms.
 8. The tail sitter aircraft asrecited in claim 1 wherein the tailboom assembly further comprises atleast four rotatably mounted tail arms.
 9. The tail sitter aircraft asrecited in claim 1 wherein the tailboom assembly further comprises anactuator assembly operable to transition the tail arms between theforward flight configuration and the landing configuration.
 10. The tailsitter aircraft as recited in claim 9 wherein the actuator assemblyfurther comprises at least one linear actuator.
 11. The tail sitteraircraft as recited in claim 9 wherein the actuator assembly furthercomprises at least one rotary actuator.
 12. The tail sitter aircraft asrecited in claim 9 wherein the actuator assembly further comprises alocking system operable to secure the tail arms in at least one of theforward flight configuration and the landing configuration.
 13. A tailsitter aircraft comprising: a fuselage having a forward portion, an aftportion and a longitudinally extending fuselage axis; first and secondfixed wings supported by the forward portion of the fuselage and thatare nonrotatable relative to the fuselage, the fixed wings providing amain lifting surface for the aircraft; a distributed propulsion systemincluding first and second rotor assemblies attached respectively to thefirst and second fixed wings, the distributed propulsion system operableto provide forward thrust during forward flight and vertical thrustduring vertical takeoff, hover and vertical landing; and a tailboomassembly extending from the aft portion of the fuselage, the tailboomassembly including a plurality of rotatably mounted tail arms havingcontrol surfaces and landing members; wherein, in a forward flightconfiguration, the tail arms are radially retracted to reduce tailsurface geometry and provide yaw and pitch control with the controlsurfaces; and wherein, in a landing configuration, the tail arms areradially extended relative to one another about the fuselage axis toform a stable ground contact base with the landing members.
 14. The tailsitter aircraft as recited in claim 13 wherein the first and secondfixed wing are generally oppositely disposed relative to the fuselage.15. The tail sitter aircraft as recited in claim 13 wherein the tailboomassembly further comprises at least three rotatably mounted tail arms.16. The tail sitter aircraft as recited in claim 13 wherein the tailboomassembly further comprises at least four rotatably mounted tail arms.17. The tail sitter aircraft as recited in claim 13 wherein the tailboomassembly further comprises an actuator assembly operable to transitionthe tail arms between the forward flight configuration and the landingconfiguration.
 18. The tail sitter aircraft as recited in claim 17wherein the actuator assembly further comprises at least one linearactuator.
 19. The tail sitter aircraft as recited in claim 17 whereinthe actuator assembly further comprises at least one rotary actuator.20. The tail sitter aircraft as recited in claim 17 wherein the actuatorassembly further comprises a locking system operable to secure the tailarms in at least one of the forward flight configuration and the landingconfiguration.